Just wanted to point to my Geek Card and say I have the exact same setup (AMD 64 3400+, 9800 Pro) but I decided to drop in 2 GB of RAM. Don't know how much better it is, but it added four inches to my ePenis!

No way to know exactly how much it will help since all programs use memory differently. I wouldn't bother just upgrading to 1 Gig unless you are having some kind of memory related problem or overrun. Most computer's have only 4 memory slots so if you get 2 512's instead of 4 256's you can pull the memory and use it again on another computer, so it won't be wasted, thus a better idea. More memory is always a good thing, just depends on how much you want to pay, mess with it.